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TRACTION MOTORS
3. TRACTION MOTORS 3.1. GENERAL
Electric mine locomotives are driven by series-wound d.c. traction motors. A few electric mine locomotives driven by single-phase induction motors with condenser starting have been built in U.S.S.R. and in Germany, but since these locomotives have not as yet come out from the experimental phase, only d.c. motors will be considered here. The speed-torque characteristic of the series-wound motors, which automatically reduce their speed with increase of the load torque, makes chese motors especially suited for traction purposes. When starting with a heavy load, the motor speed is low and so the start is very smooth. On the other hand, with light loads, the speed of the motors is high. When the degree of saturation of the magnetic circuit is low, the torque of a series-wound motor is directly proportional to the square of the current, while the torque of a shunt motor is only in direct proportion to the armature current. This means that the increase of the torque with the increase of the current is more rapid for a series-type motor than for a shunt-type motor. Comparing a shunt or a series motor of the same rating, the input of current necessary to obtain the desired starting torque will be much greater in the case of the shunt than that of the series motor. For instance, if the starting torque has to be twice the value of the rated torque, then in the case of a shunt motor the starting current will be approxi mately twice the value of the rated current, whereas for a series motor the starting current will be only about γ2 = 1.41 times the rated current. In proportion to the drop in the starting current the heat generated by the motor will decrease also. From the point of view of the power distribution, the series-type characteristic is very favourable when two or more motors are operated in parallel in a driving unit. Figure 3.1a illustrates the performance of two shunt motors operated in parallel, each having the same speed-load characteristics n =f(I), where n is the rotational speed. Let us assume that for some reason the speed of the two parallel motors in a driving unit drops by a small fraction from an initial value m to n2, while the speed of the other driving motor remains unchanged (nì). This can be caused, for example, by the slipping of the wheels of one axle or by a difference in the wear of the tyres on the road wheels. As may be seen from the diagram in Fig. 3.1a, even a very small drop in the speed of a shunt motor will cause the current intensity / to increase markedly from I\ to h. In a series-wound motor (Fig. 3.1b) a small drop of speed causes only a small change in the current intensity. The behaviour is not so favourable with greater loads when the characteristic curve becomes less steep (speed n[ and ri2, and currents I[ and I'2), but even in this range of speeds the performance is better than that of the shunt motor. The influence of the load distribution is greatest when a large traction effort is required, mainly at the start. But at the start the magnetic circuit is usually already at the saturation level owing to the increase in flux density of the magnetic field and this distorts the series characteristic of the motor. The characteristic of the series motor 36
Traction Motors
37
then becomes more rigid and more like the characteristic of a shunt motor. Then a small difference in the speeds of the particular road axles may^cause^the overloading of one traction motor and the underloading of the other.
FIG. 3.1. Distribution of loads for two traction motors (a) shunt-type motors; (b) series-type motors.
Owing to this disadvantage some designers, especially in the United States, prefer to keep the yoke well below the saturation point, but in this event the weight of the motor has to be substantially increased. To offset this inconvenience high-speed motors are being built, but this in turn often makes necessary the use of the more expensive and less efficient worm-gear, since the spur-gear ratio is usually around 1: 5 or at the most 1: 9. In Europe, where cheapness of construction is an important factor, a certain degree of rigidity of the motor characteristics under heavy loads is tolerated because then the motors can be made somewhat lighter and with lower rated motor speeds. A serious disadvantage of the series motor is that the speed of a locomotive varies greatly in the course of a service run due to variations in load or in gradient. This cuts down the average travelling speed of the train. Under such conditions motors having shunt-type characteristics increase the average travelling speed of the train. Series-wound d.c. traction motors for mining locomotives are made for the standard voltage supplies of 220 or 440 V, but motors for non-standard voltage supplies of 250 and 500 V are also available. Let us introduce here some fundamental concepts used for detailing the performance of traction motors : the one-hour and the continuous rating parameters of the motor. The one-hour rating is the maximum output at the motor shaft, measured in either horsepower or kilowatts, which the motor can carry for one hour on a stand test, with the ventilating system as in service, without exceeding the temperature limits set out in Table 3 for the particular parts of the motor. Alternatively, if the ventilating system in the stand test is different from that in service, the input of ventilating air may be not higher than that stated in the rating plate. The one-hour rating of traction motors is the same as the nominal rating. The continuous rating of a motor is the output at the motor shaft measured in either horsepower or kilowatts which the motor can carry for an unlimited period, on a stand with the ventilating system as in service, without exceeding the temperature limits permissible for the particular parts of the motor. One-hour values and continuous values can be referred also to current, rotational speed and efficiency of the motor. One-hour ratings of motors of mine locomotives used for underground service range from less than 20 to about 100 kW.
38
Underground Electric Haulage 3.2. CONSTRUCTIONAL PARTICULARS OF TRACTION MOTORS 3.2.1. MOTOR FRAME AND BEARINGS
Motor frames are made either as steel castings (Fig. 3.2) or of rolled steel plate. They are sometimes made in two parts to facilitate assembly. Frames made in one piece have flanges at both ends with machined openings to bolt down the end-plates, which are made either as steel castings or of pressed steel. Machined seats are provided in the end-plates for the armature bearings. Anti-friction bearings are of the roller or ball type. The bearing caps are provided with labyrinth seals to protect the bearings and the inside of the motor from dust and moisture and to prevent the lubricant leaking out. Bolted directly to the inside of the frame are the main and the auxiliary field pole-pieces and thus the frame acts simultaneously as the yoke of the field magnet.
FIG. 3.2. Frames of traction motors (a) cast in one piece; (b) split.
Two openings are provided in the top of the frame, one over the commutator and the other at the opposite end. The openings have hinged covers and give access to the commutator brushes and the various parts of the motor. Another opening with a hinged cover is provided at the bottom of the frame under the commutator. This serves for inspection of the bottom parts of the motor and for removing lubricants and dirt that may have penetrated from outside. The covers are lined with felt, leather, or some other material to make them dust-tight. Sometimes the openings are only covered with a piece of netting so that they form part of the ventilating system of the motor. Traction motors for mine locomotives are often of the totally enclosed type and can be either self-ventilated or have forced ventilation (for details see Section 3.2.5). Enclosing of traction motors is made necessary by the exceptionally hard conditions of underground working, but tends to increase the size and weight of the motors. Different types of traction motors are to be seen in Figs. 2.22, 2.25, 2.26, 2.29, 3.3, 3.4, 3.5 and 3.6. The one-hour rating of the motor of Fig. 3.4 is 20 kW at 220 V. The simplified diagram in Fig. 3.6 shows the cross-section through a Westinghouse motor working in conjunction with the worm-gear shown in Fig. 2.24.
3.2.2.
WINDINGS
The motors of electric locomotives for underground mine haulage usually have four field poles and four interpoles. In motors of older designs the poles were of soft
39
Traction Motors
■^^^m
FIG. 3.3. GHM motor specifically designed for mine haulage
Motor type
One-hour rating kW
A
B
C
D
E
F
G
H
/
K
GGMO20 GGMO30 GGM055
17 23 41.5
660 700 750
100 110 120
418 510 650
372 388 425
315.5 344 386.5
345 373 405
310 353 380
300 315 345
70 75 75
42 42 42
FIG. 3.4. Motor of Polish-made electric locomotive type Ld
Underground Electric Haulage
40
FIG. 3.5. Traction motor of British make
FIG. 3.6. Simplified cross-section of high-speed, 23.6 kW, 250 V, 111 A, 1200 rev/min, traction motor of American make 1—frame;
2—main field coils ; 3—armature; 4—main field pole-pieces ; 5—interpoles; 7—brushes; 8—gap under main field pole; 9—gap under interpole.
6—interpole coils ;
cast steel and the pole-pieces were built of laminated steel stampings. In present-day constructions both the main and the auxiliary field poles are built of laminated steel stampings of 0.5-1.0 mm sheet insulated either with a thin coating of varnish or with paper. This insulation serves to reduce losses in the pole-pieces due to eddy currents. The assembledfieldpoles are bolted on the interior of the motor frame. A brass plate is usually introduced between each interpole and the motor frame. The plate leaves a gap between the interpole and the frame, thus preventing the satura tion of the interpole cores and improving their efficiency under greater loads. The field coils are held in position by means of spring washers. Some details of the construction
Traction Motors
41
of field poles are shown in Fig. 3.5. The main field coils and the interpole coils are wound with wire of square cross-section covered with a double cotton or asbestos fibre insulation. More recently, coil wire is insulated with glass fibre. The coils are wound on jigs which are taken out when the operation is complete. The coil is then wrapped first with insulating board and then on top of the insulating board with im pregnated cotton webbing. The particular layers of the winding are always insulated with mica. After winding the coils are dipped in bakélite varnish, then covered with a layer of mica or glass fibre, and once again dipped in varnish to impart a good protecting shell. Thefieldpoles are wrapped in insulating board forming a layer 0.5-1.0 mm thick. The terminals of the field coils are either brazed or riveted to the ends of the field winding. This operation must be done with the utmost care. The manner of winding and insulating the interpole coils does not differ from the winding and insulation of the main field coils.
10
6
7
9
8
1
7
6
FIG. 3.7. Longitudinal section of armature of motor shown in Fig. 3.6 1—armature shaft; 2—drive transmission coupling; 3—bearing flange; 4—end-plate; 5—fan ; 6—bearing of shaft; 7—labyrinth seal; 8—ventilation duct; 9—inner clamping ring of commutator; 10—bearing box cover; 11—outer clamping ring of commutator; 12—steel bands; 13—armature core; 14—commutator insulating ring; 15—com mutator riser; 16—mica insulation; 17—outer part of armature coils.
Armatures of traction motors are shown in Figs. 3.5 and 3.7; the one shown in Fig. 3.7 belongs to the high-speed, worm-gear transmission traction motor shown in Fig. 3.6 and this accounts for its unusual length. The armature core is built up from a number of steel stampings 0.5 mm thick with a few laminations 1.5 mm thick at each end to impart greater rigidity. The stampings are mounted directly on the armature shaft. Openings punched in the stampings form axial ventilation ducts after assembly. The armature winding of traction motors is of flat copper strip, this shape of the cross-section giving a greater economy of space in the slots than would wires of cir cular cross-section. The strips are insulated with a double layer of cotton thread or with special insulation materials such as asbestos or glass fibre. The particular coils of the armature winding are wrapped with cotton webbing and impregnated. The coils are formed to the necessary shape using suitable jigs. Each slot usually holds more than one coil, thus giving a multi-layer winding. Mica paper is introduced between the particular layers as insulation. The armature winding of traction motors for mine locomotives is of the duplexwaved type, with three or more conductors per coil. The flat conductor strip is arranged vertically and in some motors also the arrangement of the coils in the slots may be vertical. The wave winding of the armature is convenient because it allows the use of only two sets of brushes in a four-pole machine, an important feature in traction mo-
42
Underground Electric Haulage
tors owing to the limited means of access to the motors. On the other hand wave winding necessitates the use of brushes with a larger cross-section. The outer parts of the coils are always bent inwards towards the armature shaft in order to economize on space (see Fig. 3.5).
3.2.3. COMMUTATOR
The commutator is built up from a number of especially shaped, hard-drawn, electrolytic copper segments clamped together to form a cylinder. The particular segments are insulated one from the other by thin layers of mica. The cylinder is clamped at both ends with rings, which hold it together. In Fig. 3.7 the inner and outer rings holding the commutator segments, as seen in cross-section view, are marked 9 and 11, respectively. The rings are covered with the insulating flange 14. The com mutator is mounted on a steel hub wedged on the armature shaft.
3.2.4. BRUSHGEAR
The brushes of traction motors are held in special holders. Generally a motor has two sets of brushes, each set being usually composed of more than one brush. Usually, traction motors have four poles. The brush holders are so mounted on the motor frame that they can all be simultaneously shifted through a certain angle to allow the adjustment of the brushes to the neutral zone. The brushes in the holders spring are loaded to press them against the commutator surface. Figure 3.8 shows a diagrammatic view of a brush holder fitted on a wave-wound, four-pole motor type GGM and the arrangement of the brush-gear on the circumference of the commutator.
3.2.5. VENTILATION
The influence of ventilation on the rating of traction motors is illustrated by the curves in Fig. 3.9: curve 1 refers to a motor with forced ventilation and curve 2 to a totally enclosed motor. The one-hour rating indicated by the straight line c is the same for both motors (48 h.p.); the continuous ratings of the motors are indicated by lines a and b. As is to be seen the continuous rating of the totally enclosed version is about 18 h.p., but if the same motor was ventilated its continuous rating would increase to about 36 h.p. The permissible loading of an enclosed motor can be increased by about 20-25 per cent by merely opening the covers over the commutator. Typical traction motors may incorporate one of the following ventilation systems: (i) external ventilation by air circulating over the frame surface, (ii) self-ventilation, (iii) forced ventilation from an independently driven blower. In the system of ventilation by external air circulation heat is removed from the motor by the relative motion of ambient air when the locomotive is moving. The efficiency of this ventilation system is very low, but even so it lowers the temperature of the motor by 15-20°C as compared to the same motor if run on an immobile stand with the same loading current.
Traction Motors
43
Self-ventilated motors have openings in the frame one at the commutator end and at the opposite end. Some motors have a fan mounted on the armature shaft to assist the circulation of air (see Fig. 3.7), this method giving better cooling than unassisted air circulation. The fan of self-ventilating motors either draws cooling air into the
FIG. 3.8. Brush holder (a) side view; (b) top view ; (c) relative position of brushes on commutator circumference.
t,hr> I 5
r 7
4 3 2
/ 0 0
a L_
b\ c j
20
J _l_
40
J
1
60
—^
HP
FIG. 3.9. Influence of ventilation on the performance of traction motors 1—ventilated motor; 2—totally enclosed motor.
motor or forces exhausts air from the machine. In order to protect the interior of the machine from dust the air intake must be fitted with a filter. Protection against dust is further improved by placing the air intake in the upper part of the motor frame. When the ventilator fan is designed to exhaust air from the motor, the intake is over
44
Underground Electric Haulage
the commutator and the outlet at the opposite end of the motor frame. Better results are obtained by exhausting hot air from the machine than by blowing in cold air, since compression always raises the air temperature by a few degrees. In self-ventilating motors the fan is mounted on the armature shaft and therefore the amount of cooling air drawn into the machine and passing through it depends upon the motor speed. When the locomotive is not moving the fan is not working and all the cooling is only due to radiation. The conditions of working of a traction motor are such that the highest current intensities occur at the start and then drop when the balancing speed is reached. On the other hand the speed of a motor increases gradually from zero at the start to a maximum when the locomotive runs at balancing speed. This characteristic of the motor is very unfavourable from the point of view of ventilation, as the cooling effect is least when the load on the motor is highest. When the locomotive is at a standstill, the cooling of motors is very slow owing to the lack of air circulation. In the present considerations the flow of air in mine workings caused by the ventilation of the mine is disregarded. The cooling of a motor during a stop of the locomotive should be sufficient to prevent the motor being overheated during the next cycle of work. The best cooling conditions are obtained by ventilation from an independently driven blower. With this system of ventilation the efficiency of cooling is almost in dependent of the speed with which the traction motor is working at any given moment. In the description above the ventilation systems were discriminated according to their external features, but they may also be considered from the point of view of the different methods of air circulation inside the motor. Air circulation in self-ventilated motors and in motors with independent forced ventilation may be classified as of (i) the single-flow type, (ii) the double-flow type. With the single-flow system air flows only between the interior surfaces of the motor and with the double-flow system air also passes through ducts provided for this pur pose in the armature core (Fig. 3.7). The double-flow system of air circulation can be further subdivided into circulation in series, circulation in parallel and a combination of these two types. Circulation in series consists in passing the cooling air first through the ducts in the armature core and then between the other interior surfaces of the motor or in the reverse direction. With the parallel circulation, cooling air is divided into two streams, one passing through the ducts in the armature core and the other between the field magnet and the armature. At present this type of air circulation is regarded as the most effective and is the most widely applied. In traction motors of low and medium rating, and such are the motors used on mine locomotives, limitations of space often make it difficult to provide ventilation ducts in the armature core, so that ventilation has to be of the single-flow type. Also owing to the limited space the traction motors of mine locomotives very seldom have axle mounted fans. The different systems of ventilation applied in traction motors are compared in Table 1. Since within the limits of normal service loads the efficiency of motors varies over a wide range, the rating ratios listed in Table 1 can, if necessary, be replaced by current ratios. The motors used in mine locomotives are most often of the totally enclosed type.
Traction Motors
45
TABLE 1. THE EFFICIENCY OF DIFFERENT METHODS OF VENTILATION OF TRACTION MOTORS
No.
Method of ventilation
Ratio of continu Ratio of conti ous rating of nuous to onemotor with given hour rating for ventilation Pao(v) given ventilation system to contin system and nom uous rating of inal voltage at totally enclosed collector motor Poo(o) PoolPh Λχ)(ϋ)ΑΡοο(θ)
Ratio of onehour rating of motor with given ventilation P/,(y) system to onehour rating of totally enclosed motor Pfc(o)
Range of application
Ph{v)/Ph(o)
1
Motor totally en closed with external ventilation
1
0.35-0.45
—
Motors of various ratings mainly on underground mine locomotives
2
Self-ventilated mo tor with single flow circulation and fan on shaft
1.5-1.7
0.55-0.62
1.08-1.15
Motors of 40-80 kW
3
Self-ventilated mo tor with single-flow circulation and no fan
1.2-1.25
0.4-0.5
1.05-1.10
Mainly motors of underground mine locomotives
4
Self-ventilated mo tor with doubleflow, series circula tion and fan on shaft
1.6-1.75
0.57-0.66
1.05-1.15
Motors of various ratings
5
Self-ventilated mo tor with doubleflow, parallel circu lation and fan on shaft
1.7-2.0
0.65-0.78
1.05-1.15
Motors of various ratings
6
Self-ventilated mo tor with doubleflow, parallel circu lation and fan on shaft (high-speed motors)
2.0-2.75
0.82-0.95
1.05-1.20
Motors with nomi nal speed over 1000 rev/min
7
Self-ventilated mo tor with combined (series and parallel) circulation and fan on shaft
1.7-1.95
0.64-0.77
1.05-1.15
Motors of various ratings
8
Force-ventilated motor
2.2-2.5
0.8-0.9
1.05-1.20
Mainly motors of higher ratings (over 40 kW) for surface traction
9
Force-ventilated motor
1.33-1.50
0.5-0.6
1.05-1.08
Motors of under ground mine loco motives from 15kWup
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Underground Electric Haulage
For such motors the ratio of the continuous rating to the one-hour rating may be assumed to be about 0.35-0.45. Some of the types of locomotives manufactured by the Westinghouse Company are fitted with high-speed, worm-gear transmission motors with the fan mounted on the armature shaft (Fig. 3.7). The motors of this type belong to group No. 6 according to the list in Table 1, i.e. to the group of self-ventilated motors. In many traction motors with an up-to-date design the ventilation system is of the independent forced type, cooling air being supplied by a small fan driven by a separate low-power motor. The blower unit is then located at some distance from the traction motors and is connected with the outlets of the motor frames (in the case of exhaust ventilation) by flexible ducts. Usually, one motor drives two ventilator fans. The rating of the blower motor is much lower than the gain in the rating of the traction motors obtained by ventilation. When long non-stop runs are the rule the time of uninterrupted work of the traction motors is too long for the motors to cool down sufficiently during the stops. In such instances independent forced ventilation greatly improves the efficiency. Under the conditions of underground working in mines the greatest gains in effi ciency are obtained when the ratios of the blower rating to the rating of the traction motors has certain strictly defined values. The usual practice is not to utilize all the increased power that is made available in the traction motors. When the overloads are short-lasting, ventilation has little significance. The main advantages of ventilation appear in the case of continuous loads sustained for long periods. It is easy to calculate from the data listed in Table 1 that ventilation can increase the one-hour ratings of motors by 5-20 per cent. The data for the ventilation system listed as No. 8 in Table 1 refer to the rating of normal motors used on the surface when the additional power made available by independent forced ventilation is fully utilized, and the data for the system listed as No. 9 refer to traction motors used on mine locomotives working underground. In the motors of the latter type independent forced ventilation increases the continuous rating 1.33-1.5 times and the one-hour rating only by about 5 per cent. Independent forced ventilation cannot be applied in fiery mines as the dust would penetrate to the interior of the motor. The data listed in Table 2 show how the different methods of ventilation influence the rating of a traction motor. All the data refer to the same motor. The overload capacity of a motor is defined by the ratio of the continuous rating to the one-hour rating of the motor. The value of this ratio depends on the heat capacity and the rate of cooling. As the light armatures of high-speed motors have a low heat capacity and as under the conditions prevailing in mines, where high overloads have to be applied at rela tively short intervals, the traction motors must have a high heat capacity, low-speed traction motors are generally preferred for mine locomotives. The rating of the blower motor for the independent ventilation of traction motors can be calculated in a simplified way. Mention has already been made that the usual practice is to have a separate ventilator for each traction motor and to drive them both with one low-rating motor. The volume of cooling air that must flow through the traction motor depends on the power losses in the motor. The amount of heat Q generated in the motor by the osses Δ Ρ during time at is Q = 0.24APdt cal. (3.1)
Traction Motors
47
TABLE 2. INFLUENCE OF THE VENTILATION SYSTEM ON THE RATING OF TRACTION MOTORS
(all the data in this table refer to the same motor)
No.
System of ventilation
1
Double-flow, parallel, self-venti lation Single-flow self-ventilation (air ducts in armature blocked) External ventilation (all openings in motor frame closed) External ventilation (fan removed, all openings in motor frame closed)
2 3 4
Time of heat-] ing to con Time constant of heating stant temper ature T hours / hours
One-hour rating Ph h.p.
Continuous rating Poo h.p.
112
75
5
more than 1J
107
65
7
more than 1J
101
47
8
more than 2
100
45
10
about 2J
When making approximate calculations radiation and convection of heat are disregarded on the assumption that all the heat generated is carried away by the flow of cooling air. The cooling air can carry away only a part of the heat generated in the motors. If all the heat were to be carried away, the volume of the air flow would have to be much greater, but then the losses due to air friction and, hence, the necessary pressure would rise rapidly and so would the required rating of the blower motor. (The rise of the blower motor rating would have to be approximately proportional to the cube of the volume of air flowing through the traction motor.) This means that the losses in the blower and in the blower motor would reach a point at which they would offset the advantages of ventilation and that the efficiency of ventilation near the maximum pos sible volume of air flow would be low. For practical purposes independent forced venti lation is designed to increase the continuous rating of the traction motor to about 65 per cent of the one-hour rating, the simultaneous rise of the one-hour rating being about 5 per cent (see Table 1). When the volume of cooling air blown in or exhausted by the ventilator is V0 (m3/sec) and the effective pressure or vacuum is h (mm of water column), then the rating of the blower motor Pw is calculated from the formula 102 η where η is the resultant efficiency of the blower and the blower motor; the value of η may be assumed as 0.5-0.6. Data obtained under actual working conditions show that the value of h ranges from 16 to 20 mm of water column for traction motors with one-hour ratings up to 50 kW and from 16 to 25 mm of water column for motors with one-hour ratings from 50 to 100 kW. The losses in the traction motor necessary to raise the temperature of 1 m3 of air by #°C are calculated on the assumption that the density of air is 1.293 kg/m 3 , the specific heat of air is 0.237, and the electrical equivalent of a calorie is 4.2W sec. Then N= 1293x0.237x4.2 Û = 1280 ê Wsec. (3.3) Assuming further that the rise of temperature of the cooling air is # = 20°C, we have N=25.6 kWsec. (3.4)
Underground Electric Haulage
48
This means that for every 25.6 kW of losses in the motor the stream of cooling air must be 1 m3/sec, if the rise of temperature is to be 20°C. In traction motors used on the surface the temperature rise is usually 40-50°C, but to keep on the safe side lower rises in temperature of only 20-30°C are accepted for locomotives working under ground. On passing through the blower the temperature of air rises by about 3-5°C. This circumstance makes exhaust ventilation more efficient than forced ventilation: in the former case the temperature of the cooling air reaching the interior of the traction motor is the same as the ambient temperature. The following example shows how to calculate the performance of an independent blower for cooling a traction motor. One-hour rating of the traction motor with no ventilation Continuous rating of the traction motor with no ventilation One-hour efficiency Continuous efficiency
Ph = Pw = ηΗ = η^ =
41.5 kW 19 kW 0.88 0.87
From the data in Table 1 we can assume that independent forced ventilation in creases the one-hour rating of the traction motor by 5 per cent. Then Ph{v) = 41.5x1.05 = 43.6 kW. The continuous rating can then be calculated approximately as Poo(v) = 0.55 PhM = 0.55x43.6 = 25 kW. The efficiency of the traction motor will then be Vm
=0.88,
ifeo(,) = 0.90. The total losses in the output of the traction motor are therefore
AP
^ = (-^~l) p^ = ( w -l)2S = 2 · 7 5
kw
·
The volume of cooling air that must be passed through the traction motor to carry off this amount of heat—see eqn. (3.4)—is calculated as Tz
2 7 5
·
^o = "2^-g- ^ 0 . 1 1 m3/sec,
when it is assumed that the rise of temperature is 20°C. When h = 20 mm of water column and η = 0.5, the output of the ventilator is calculated from formula (3.2): Fw
~
_ 0.11X20 102x0.5 " ü e ü 4 i
kW
·
As is to be seen forced ventilation from an independently driven ventilator substantially increases the output of the traction motor. The input power of the blower motor, calculated as the percentage of the one-hour input of the traction motor, is ,™ 0.043 Λ1 Pw = 100 -TfT" ~ ° · 1
per
cent
Traction Motors
49
The temperatures to which motors and their different parts can be allowed to rise depend on the nature of the winding insulation. These temperatures are defined in electrical engineering standards (see Tables 3, 4, and 5). TABLE 3. PERMISSIBLE TEMPERATURE RISE OF TRACTION MOTORS ACCORDING TO BRITISH (B.E.S.A.) AND AMERICAN (A.I.E.E.) STANDARDS
Method of Type of enclo determination of tempera sure ture
Item
B<2)
Ad)
B<2)
85
105
80
95
65
80
110
130
95
115
90
105
75
90
Ventilated
Thermo meter
90 B.E.S.A. 80 A.I.E.E.
95
65
80
Totally enclosed
Thermo meter
95 B.E.S.A. 90 A.I.E.E.
105
75
90
Ventilated
Thermo meter
95
110
80
95
Thermo Totally enclosed meter
105
120
90
105
Thermo meter Resistance
Totally enclosed
Commutator
Ad)
Continuous rating
120
Ventilated
Cores and mechanical parts in contact with or adjacent to insula tion
One-hour rating
100
Resistance
Armature and field windings
Limiting temperature rise #°C
Thermo meter
(î) "A" denotes "Class A" insulation, which is cotton, silk, paper and similar organic materials when impregnated ; also enamel as applied to conductors. (2) "g" denotes "Class B" insulation, which is inorganic materials, such as mica and asbestos in built-up form combined with binding substances. If Class A material is used in small quantities in conjunction for structural purposes only, the combined material may be considered as Class B, pro vided that the electrical and mechanical properties of the insulated windings are not impaired by the application of the temperature permitted for Class B material. TABLE 4. PERMISSIBLE TEMPERATURE RISE (DEGREES C) OF TRACTION MOTORS ACCORDING TO INTERNATIONAL STANDARDS
Continuous rating
measured measured measured measured with resist with thermo with resist with thermo meter ance method meter ance method
Item
Armature and field windings
One-hour rating
Class A insulation
85
65
100
75
Class B insulation
105
85
120
95
—
85
—
90
Tie rings and com mutators
Notes
ambient tempera ture 25°C
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Underground Electric Haulage
TABLE 5. PERMISSIBLE TEMPERATURE RISE (DEGREES C) OF TRACTION MOTORS ACCORDING TO POLISH
STANDARD PN-57/E-06001
Continuous rating Measured with resis tance me thod
Item
Ambient Armature winding Field winding
Measured with thermo meter
temperature
Class A insulation*2) Class B insulation Class A insulation*2* Class B insulation
Short-time duty (one-hour rating) Measured Measured with resis tance me with thermo meter thod
25001>
85
65
100
75
120
105
120
105
85
65
100
75
130
100
130
100
90
Commutator
90
w If the temperature of cooling air is higher than 25°C the permitted temperature rise of the particular parts of the motors must be accordingly reduced. <2> The classes of insulation according to Polish Standard PN-55/E-06000 are: Class A insulation is cotton, silk, paper and other organic materials saturated with a binding substance or permanently immersed in a liquid dielectric; Class B insulation is mica, asbestos and similar inorganic materials in built-up form combined with binding substances.
According to the definitions set forth in the Polish Standards PN-55/E-06000 and PN-57/E-06001 the rise of temperature equals the difference between the temperature of the particular part of the motor and the temperature of the cooling air when the work of the motor is continuous or intermittent, or the difference between the temper ature of the particular parts of the motor as measured at the beginning and the end of the period of work when the motor is running on cycles of short duration. Polish Standard PN-57/E-06001 specifies that measurements of the temperature rise made with a thermometer may be accepted only as approximate. So far as the permissible temperatures are concerned the most important parts of the motor are the windings. The rise of temperature (in degrees Centigrade) of the windings can be calculated from the measured resistance according to the formula M
= ^L_J?£ = (234.5+0 c )-(# w -0 c )
where Δ# = the temperature increase to be calculated, êc = the temperature of winding when cold, &m = the temperature of cooling medium, Rc = the resistance of winding when cold, Rh = the resistance of winding when hot. In most cases u
m —
u
c·
Traction Motors
51
3.3. WEIGHTS AND RATINGS OF TRACTION MOTORS
The weight of traction motors used for driving electric mine locomotives for under ground duties ranges from about 300 to about 1000 kg. The weight of the motor depends on its rating and its speed, i.e. on the torque. Electric locomotives of the same weight may be fitted with lighter, lower-rating motors or with heavier, higher-rating motors. Generally, mine locomotives of European make are fitted with motors having lower-ratings per ton of the mine locomotive weight than mine locomotives of American make. The ratio of the rating of motors to the locomotive weight for mine locomotives of European make varies from 4.5 to 6.9 kW per ton at one-hour rated speed of 13 km/hr, while the same ratio for mine locomotives of American make varies from 7.3 kW per ton at one-hour rated speed of 13 km/hr to 14.5 kW per ton at one-hour rated speed of 19 km/hr. The surplus power of the motors allows greater acceleration and more rapid start ing, but also has certain disadvantages as, for example, the slipping of wheels on the rails.
Electric mine locomotives are driven by series-wound d.c. traction motors. A few electric mine locomotives driven by single-phase induction motors with condenser starting have been built in U.S.S.R. and in Germany, but since these locomotives have not as yet come out from the experimental phase, only d.c. motors will be considered here. The speed-torque characteristic of the series-wound motors, which automatically reduce their speed with increase of the load torque, makes chese motors especially suited for traction purposes. When starting with a heavy load, the motor speed is low and so the start is very smooth. On the other hand, with light loads, the speed of the motors is high. When the degree of saturation of the magnetic circuit is low, the torque of a series-wound motor is directly proportional to the square of the current, while the torque of a shunt motor is only in direct proportion to the armature current. This means that the increase of the torque with the increase of the current is more rapid for a series-type motor than for a shunt-type motor. Comparing a shunt or a series motor of the same rating, the input of current necessary to obtain the desired starting torque will be much greater in the case of the shunt than that of the series motor. For instance, if the starting torque has to be twice the value of the rated torque, then in the case of a shunt motor the starting current will be approxi mately twice the value of the rated current, whereas for a series motor the starting current will be only about γ2 = 1.41 times the rated current. In proportion to the drop in the starting current the heat generated by the motor will decrease also. From the point of view of the power distribution, the series-type characteristic is very favourable when two or more motors are operated in parallel in a driving unit. Figure 3.1a illustrates the performance of two shunt motors operated in parallel, each having the same speed-load characteristics n =f(I), where n is the rotational speed. Let us assume that for some reason the speed of the two parallel motors in a driving unit drops by a small fraction from an initial value m to n2, while the speed of the other driving motor remains unchanged (nì). This can be caused, for example, by the slipping of the wheels of one axle or by a difference in the wear of the tyres on the road wheels. As may be seen from the diagram in Fig. 3.1a, even a very small drop in the speed of a shunt motor will cause the current intensity / to increase markedly from I\ to h. In a series-wound motor (Fig. 3.1b) a small drop of speed causes only a small change in the current intensity. The behaviour is not so favourable with greater loads when the characteristic curve becomes less steep (speed n[ and ri2, and currents I[ and I'2), but even in this range of speeds the performance is better than that of the shunt motor. The influence of the load distribution is greatest when a large traction effort is required, mainly at the start. But at the start the magnetic circuit is usually already at the saturation level owing to the increase in flux density of the magnetic field and this distorts the series characteristic of the motor. The characteristic of the series motor 36
Traction Motors
37
then becomes more rigid and more like the characteristic of a shunt motor. Then a small difference in the speeds of the particular road axles may^cause^the overloading of one traction motor and the underloading of the other.
FIG. 3.1. Distribution of loads for two traction motors (a) shunt-type motors; (b) series-type motors.
Owing to this disadvantage some designers, especially in the United States, prefer to keep the yoke well below the saturation point, but in this event the weight of the motor has to be substantially increased. To offset this inconvenience high-speed motors are being built, but this in turn often makes necessary the use of the more expensive and less efficient worm-gear, since the spur-gear ratio is usually around 1: 5 or at the most 1: 9. In Europe, where cheapness of construction is an important factor, a certain degree of rigidity of the motor characteristics under heavy loads is tolerated because then the motors can be made somewhat lighter and with lower rated motor speeds. A serious disadvantage of the series motor is that the speed of a locomotive varies greatly in the course of a service run due to variations in load or in gradient. This cuts down the average travelling speed of the train. Under such conditions motors having shunt-type characteristics increase the average travelling speed of the train. Series-wound d.c. traction motors for mining locomotives are made for the standard voltage supplies of 220 or 440 V, but motors for non-standard voltage supplies of 250 and 500 V are also available. Let us introduce here some fundamental concepts used for detailing the performance of traction motors : the one-hour and the continuous rating parameters of the motor. The one-hour rating is the maximum output at the motor shaft, measured in either horsepower or kilowatts, which the motor can carry for one hour on a stand test, with the ventilating system as in service, without exceeding the temperature limits set out in Table 3 for the particular parts of the motor. Alternatively, if the ventilating system in the stand test is different from that in service, the input of ventilating air may be not higher than that stated in the rating plate. The one-hour rating of traction motors is the same as the nominal rating. The continuous rating of a motor is the output at the motor shaft measured in either horsepower or kilowatts which the motor can carry for an unlimited period, on a stand with the ventilating system as in service, without exceeding the temperature limits permissible for the particular parts of the motor. One-hour values and continuous values can be referred also to current, rotational speed and efficiency of the motor. One-hour ratings of motors of mine locomotives used for underground service range from less than 20 to about 100 kW.
38
Underground Electric Haulage 3.2. CONSTRUCTIONAL PARTICULARS OF TRACTION MOTORS 3.2.1. MOTOR FRAME AND BEARINGS
Motor frames are made either as steel castings (Fig. 3.2) or of rolled steel plate. They are sometimes made in two parts to facilitate assembly. Frames made in one piece have flanges at both ends with machined openings to bolt down the end-plates, which are made either as steel castings or of pressed steel. Machined seats are provided in the end-plates for the armature bearings. Anti-friction bearings are of the roller or ball type. The bearing caps are provided with labyrinth seals to protect the bearings and the inside of the motor from dust and moisture and to prevent the lubricant leaking out. Bolted directly to the inside of the frame are the main and the auxiliary field pole-pieces and thus the frame acts simultaneously as the yoke of the field magnet.
FIG. 3.2. Frames of traction motors (a) cast in one piece; (b) split.
Two openings are provided in the top of the frame, one over the commutator and the other at the opposite end. The openings have hinged covers and give access to the commutator brushes and the various parts of the motor. Another opening with a hinged cover is provided at the bottom of the frame under the commutator. This serves for inspection of the bottom parts of the motor and for removing lubricants and dirt that may have penetrated from outside. The covers are lined with felt, leather, or some other material to make them dust-tight. Sometimes the openings are only covered with a piece of netting so that they form part of the ventilating system of the motor. Traction motors for mine locomotives are often of the totally enclosed type and can be either self-ventilated or have forced ventilation (for details see Section 3.2.5). Enclosing of traction motors is made necessary by the exceptionally hard conditions of underground working, but tends to increase the size and weight of the motors. Different types of traction motors are to be seen in Figs. 2.22, 2.25, 2.26, 2.29, 3.3, 3.4, 3.5 and 3.6. The one-hour rating of the motor of Fig. 3.4 is 20 kW at 220 V. The simplified diagram in Fig. 3.6 shows the cross-section through a Westinghouse motor working in conjunction with the worm-gear shown in Fig. 2.24.
3.2.2.
WINDINGS
The motors of electric locomotives for underground mine haulage usually have four field poles and four interpoles. In motors of older designs the poles were of soft
39
Traction Motors
■^^^m
FIG. 3.3. GHM motor specifically designed for mine haulage
Motor type
One-hour rating kW
A
B
C
D
E
F
G
H
/
K
GGMO20 GGMO30 GGM055
17 23 41.5
660 700 750
100 110 120
418 510 650
372 388 425
315.5 344 386.5
345 373 405
310 353 380
300 315 345
70 75 75
42 42 42
FIG. 3.4. Motor of Polish-made electric locomotive type Ld
Underground Electric Haulage
40
FIG. 3.5. Traction motor of British make
FIG. 3.6. Simplified cross-section of high-speed, 23.6 kW, 250 V, 111 A, 1200 rev/min, traction motor of American make 1—frame;
2—main field coils ; 3—armature; 4—main field pole-pieces ; 5—interpoles; 7—brushes; 8—gap under main field pole; 9—gap under interpole.
6—interpole coils ;
cast steel and the pole-pieces were built of laminated steel stampings. In present-day constructions both the main and the auxiliary field poles are built of laminated steel stampings of 0.5-1.0 mm sheet insulated either with a thin coating of varnish or with paper. This insulation serves to reduce losses in the pole-pieces due to eddy currents. The assembledfieldpoles are bolted on the interior of the motor frame. A brass plate is usually introduced between each interpole and the motor frame. The plate leaves a gap between the interpole and the frame, thus preventing the satura tion of the interpole cores and improving their efficiency under greater loads. The field coils are held in position by means of spring washers. Some details of the construction
Traction Motors
41
of field poles are shown in Fig. 3.5. The main field coils and the interpole coils are wound with wire of square cross-section covered with a double cotton or asbestos fibre insulation. More recently, coil wire is insulated with glass fibre. The coils are wound on jigs which are taken out when the operation is complete. The coil is then wrapped first with insulating board and then on top of the insulating board with im pregnated cotton webbing. The particular layers of the winding are always insulated with mica. After winding the coils are dipped in bakélite varnish, then covered with a layer of mica or glass fibre, and once again dipped in varnish to impart a good protecting shell. Thefieldpoles are wrapped in insulating board forming a layer 0.5-1.0 mm thick. The terminals of the field coils are either brazed or riveted to the ends of the field winding. This operation must be done with the utmost care. The manner of winding and insulating the interpole coils does not differ from the winding and insulation of the main field coils.
10
6
7
9
8
1
7
6
FIG. 3.7. Longitudinal section of armature of motor shown in Fig. 3.6 1—armature shaft; 2—drive transmission coupling; 3—bearing flange; 4—end-plate; 5—fan ; 6—bearing of shaft; 7—labyrinth seal; 8—ventilation duct; 9—inner clamping ring of commutator; 10—bearing box cover; 11—outer clamping ring of commutator; 12—steel bands; 13—armature core; 14—commutator insulating ring; 15—com mutator riser; 16—mica insulation; 17—outer part of armature coils.
Armatures of traction motors are shown in Figs. 3.5 and 3.7; the one shown in Fig. 3.7 belongs to the high-speed, worm-gear transmission traction motor shown in Fig. 3.6 and this accounts for its unusual length. The armature core is built up from a number of steel stampings 0.5 mm thick with a few laminations 1.5 mm thick at each end to impart greater rigidity. The stampings are mounted directly on the armature shaft. Openings punched in the stampings form axial ventilation ducts after assembly. The armature winding of traction motors is of flat copper strip, this shape of the cross-section giving a greater economy of space in the slots than would wires of cir cular cross-section. The strips are insulated with a double layer of cotton thread or with special insulation materials such as asbestos or glass fibre. The particular coils of the armature winding are wrapped with cotton webbing and impregnated. The coils are formed to the necessary shape using suitable jigs. Each slot usually holds more than one coil, thus giving a multi-layer winding. Mica paper is introduced between the particular layers as insulation. The armature winding of traction motors for mine locomotives is of the duplexwaved type, with three or more conductors per coil. The flat conductor strip is arranged vertically and in some motors also the arrangement of the coils in the slots may be vertical. The wave winding of the armature is convenient because it allows the use of only two sets of brushes in a four-pole machine, an important feature in traction mo-
42
Underground Electric Haulage
tors owing to the limited means of access to the motors. On the other hand wave winding necessitates the use of brushes with a larger cross-section. The outer parts of the coils are always bent inwards towards the armature shaft in order to economize on space (see Fig. 3.5).
3.2.3. COMMUTATOR
The commutator is built up from a number of especially shaped, hard-drawn, electrolytic copper segments clamped together to form a cylinder. The particular segments are insulated one from the other by thin layers of mica. The cylinder is clamped at both ends with rings, which hold it together. In Fig. 3.7 the inner and outer rings holding the commutator segments, as seen in cross-section view, are marked 9 and 11, respectively. The rings are covered with the insulating flange 14. The com mutator is mounted on a steel hub wedged on the armature shaft.
3.2.4. BRUSHGEAR
The brushes of traction motors are held in special holders. Generally a motor has two sets of brushes, each set being usually composed of more than one brush. Usually, traction motors have four poles. The brush holders are so mounted on the motor frame that they can all be simultaneously shifted through a certain angle to allow the adjustment of the brushes to the neutral zone. The brushes in the holders spring are loaded to press them against the commutator surface. Figure 3.8 shows a diagrammatic view of a brush holder fitted on a wave-wound, four-pole motor type GGM and the arrangement of the brush-gear on the circumference of the commutator.
3.2.5. VENTILATION
The influence of ventilation on the rating of traction motors is illustrated by the curves in Fig. 3.9: curve 1 refers to a motor with forced ventilation and curve 2 to a totally enclosed motor. The one-hour rating indicated by the straight line c is the same for both motors (48 h.p.); the continuous ratings of the motors are indicated by lines a and b. As is to be seen the continuous rating of the totally enclosed version is about 18 h.p., but if the same motor was ventilated its continuous rating would increase to about 36 h.p. The permissible loading of an enclosed motor can be increased by about 20-25 per cent by merely opening the covers over the commutator. Typical traction motors may incorporate one of the following ventilation systems: (i) external ventilation by air circulating over the frame surface, (ii) self-ventilation, (iii) forced ventilation from an independently driven blower. In the system of ventilation by external air circulation heat is removed from the motor by the relative motion of ambient air when the locomotive is moving. The efficiency of this ventilation system is very low, but even so it lowers the temperature of the motor by 15-20°C as compared to the same motor if run on an immobile stand with the same loading current.
Traction Motors
43
Self-ventilated motors have openings in the frame one at the commutator end and at the opposite end. Some motors have a fan mounted on the armature shaft to assist the circulation of air (see Fig. 3.7), this method giving better cooling than unassisted air circulation. The fan of self-ventilating motors either draws cooling air into the
FIG. 3.8. Brush holder (a) side view; (b) top view ; (c) relative position of brushes on commutator circumference.
t,hr> I 5
r 7
4 3 2
/ 0 0
a L_
b\ c j
20
J _l_
40
J
1
60
—^
HP
FIG. 3.9. Influence of ventilation on the performance of traction motors 1—ventilated motor; 2—totally enclosed motor.
motor or forces exhausts air from the machine. In order to protect the interior of the machine from dust the air intake must be fitted with a filter. Protection against dust is further improved by placing the air intake in the upper part of the motor frame. When the ventilator fan is designed to exhaust air from the motor, the intake is over
44
Underground Electric Haulage
the commutator and the outlet at the opposite end of the motor frame. Better results are obtained by exhausting hot air from the machine than by blowing in cold air, since compression always raises the air temperature by a few degrees. In self-ventilating motors the fan is mounted on the armature shaft and therefore the amount of cooling air drawn into the machine and passing through it depends upon the motor speed. When the locomotive is not moving the fan is not working and all the cooling is only due to radiation. The conditions of working of a traction motor are such that the highest current intensities occur at the start and then drop when the balancing speed is reached. On the other hand the speed of a motor increases gradually from zero at the start to a maximum when the locomotive runs at balancing speed. This characteristic of the motor is very unfavourable from the point of view of ventilation, as the cooling effect is least when the load on the motor is highest. When the locomotive is at a standstill, the cooling of motors is very slow owing to the lack of air circulation. In the present considerations the flow of air in mine workings caused by the ventilation of the mine is disregarded. The cooling of a motor during a stop of the locomotive should be sufficient to prevent the motor being overheated during the next cycle of work. The best cooling conditions are obtained by ventilation from an independently driven blower. With this system of ventilation the efficiency of cooling is almost in dependent of the speed with which the traction motor is working at any given moment. In the description above the ventilation systems were discriminated according to their external features, but they may also be considered from the point of view of the different methods of air circulation inside the motor. Air circulation in self-ventilated motors and in motors with independent forced ventilation may be classified as of (i) the single-flow type, (ii) the double-flow type. With the single-flow system air flows only between the interior surfaces of the motor and with the double-flow system air also passes through ducts provided for this pur pose in the armature core (Fig. 3.7). The double-flow system of air circulation can be further subdivided into circulation in series, circulation in parallel and a combination of these two types. Circulation in series consists in passing the cooling air first through the ducts in the armature core and then between the other interior surfaces of the motor or in the reverse direction. With the parallel circulation, cooling air is divided into two streams, one passing through the ducts in the armature core and the other between the field magnet and the armature. At present this type of air circulation is regarded as the most effective and is the most widely applied. In traction motors of low and medium rating, and such are the motors used on mine locomotives, limitations of space often make it difficult to provide ventilation ducts in the armature core, so that ventilation has to be of the single-flow type. Also owing to the limited space the traction motors of mine locomotives very seldom have axle mounted fans. The different systems of ventilation applied in traction motors are compared in Table 1. Since within the limits of normal service loads the efficiency of motors varies over a wide range, the rating ratios listed in Table 1 can, if necessary, be replaced by current ratios. The motors used in mine locomotives are most often of the totally enclosed type.
Traction Motors
45
TABLE 1. THE EFFICIENCY OF DIFFERENT METHODS OF VENTILATION OF TRACTION MOTORS
No.
Method of ventilation
Ratio of continu Ratio of conti ous rating of nuous to onemotor with given hour rating for ventilation Pao(v) given ventilation system to contin system and nom uous rating of inal voltage at totally enclosed collector motor Poo(o) PoolPh Λχ)(ϋ)ΑΡοο(θ)
Ratio of onehour rating of motor with given ventilation P/,(y) system to onehour rating of totally enclosed motor Pfc(o)
Range of application
Ph{v)/Ph(o)
1
Motor totally en closed with external ventilation
1
0.35-0.45
—
Motors of various ratings mainly on underground mine locomotives
2
Self-ventilated mo tor with single flow circulation and fan on shaft
1.5-1.7
0.55-0.62
1.08-1.15
Motors of 40-80 kW
3
Self-ventilated mo tor with single-flow circulation and no fan
1.2-1.25
0.4-0.5
1.05-1.10
Mainly motors of underground mine locomotives
4
Self-ventilated mo tor with doubleflow, series circula tion and fan on shaft
1.6-1.75
0.57-0.66
1.05-1.15
Motors of various ratings
5
Self-ventilated mo tor with doubleflow, parallel circu lation and fan on shaft
1.7-2.0
0.65-0.78
1.05-1.15
Motors of various ratings
6
Self-ventilated mo tor with doubleflow, parallel circu lation and fan on shaft (high-speed motors)
2.0-2.75
0.82-0.95
1.05-1.20
Motors with nomi nal speed over 1000 rev/min
7
Self-ventilated mo tor with combined (series and parallel) circulation and fan on shaft
1.7-1.95
0.64-0.77
1.05-1.15
Motors of various ratings
8
Force-ventilated motor
2.2-2.5
0.8-0.9
1.05-1.20
Mainly motors of higher ratings (over 40 kW) for surface traction
9
Force-ventilated motor
1.33-1.50
0.5-0.6
1.05-1.08
Motors of under ground mine loco motives from 15kWup
46
Underground Electric Haulage
For such motors the ratio of the continuous rating to the one-hour rating may be assumed to be about 0.35-0.45. Some of the types of locomotives manufactured by the Westinghouse Company are fitted with high-speed, worm-gear transmission motors with the fan mounted on the armature shaft (Fig. 3.7). The motors of this type belong to group No. 6 according to the list in Table 1, i.e. to the group of self-ventilated motors. In many traction motors with an up-to-date design the ventilation system is of the independent forced type, cooling air being supplied by a small fan driven by a separate low-power motor. The blower unit is then located at some distance from the traction motors and is connected with the outlets of the motor frames (in the case of exhaust ventilation) by flexible ducts. Usually, one motor drives two ventilator fans. The rating of the blower motor is much lower than the gain in the rating of the traction motors obtained by ventilation. When long non-stop runs are the rule the time of uninterrupted work of the traction motors is too long for the motors to cool down sufficiently during the stops. In such instances independent forced ventilation greatly improves the efficiency. Under the conditions of underground working in mines the greatest gains in effi ciency are obtained when the ratios of the blower rating to the rating of the traction motors has certain strictly defined values. The usual practice is not to utilize all the increased power that is made available in the traction motors. When the overloads are short-lasting, ventilation has little significance. The main advantages of ventilation appear in the case of continuous loads sustained for long periods. It is easy to calculate from the data listed in Table 1 that ventilation can increase the one-hour ratings of motors by 5-20 per cent. The data for the ventilation system listed as No. 8 in Table 1 refer to the rating of normal motors used on the surface when the additional power made available by independent forced ventilation is fully utilized, and the data for the system listed as No. 9 refer to traction motors used on mine locomotives working underground. In the motors of the latter type independent forced ventilation increases the continuous rating 1.33-1.5 times and the one-hour rating only by about 5 per cent. Independent forced ventilation cannot be applied in fiery mines as the dust would penetrate to the interior of the motor. The data listed in Table 2 show how the different methods of ventilation influence the rating of a traction motor. All the data refer to the same motor. The overload capacity of a motor is defined by the ratio of the continuous rating to the one-hour rating of the motor. The value of this ratio depends on the heat capacity and the rate of cooling. As the light armatures of high-speed motors have a low heat capacity and as under the conditions prevailing in mines, where high overloads have to be applied at rela tively short intervals, the traction motors must have a high heat capacity, low-speed traction motors are generally preferred for mine locomotives. The rating of the blower motor for the independent ventilation of traction motors can be calculated in a simplified way. Mention has already been made that the usual practice is to have a separate ventilator for each traction motor and to drive them both with one low-rating motor. The volume of cooling air that must flow through the traction motor depends on the power losses in the motor. The amount of heat Q generated in the motor by the osses Δ Ρ during time at is Q = 0.24APdt cal. (3.1)
Traction Motors
47
TABLE 2. INFLUENCE OF THE VENTILATION SYSTEM ON THE RATING OF TRACTION MOTORS
(all the data in this table refer to the same motor)
No.
System of ventilation
1
Double-flow, parallel, self-venti lation Single-flow self-ventilation (air ducts in armature blocked) External ventilation (all openings in motor frame closed) External ventilation (fan removed, all openings in motor frame closed)
2 3 4
Time of heat-] ing to con Time constant of heating stant temper ature T hours / hours
One-hour rating Ph h.p.
Continuous rating Poo h.p.
112
75
5
more than 1J
107
65
7
more than 1J
101
47
8
more than 2
100
45
10
about 2J
When making approximate calculations radiation and convection of heat are disregarded on the assumption that all the heat generated is carried away by the flow of cooling air. The cooling air can carry away only a part of the heat generated in the motors. If all the heat were to be carried away, the volume of the air flow would have to be much greater, but then the losses due to air friction and, hence, the necessary pressure would rise rapidly and so would the required rating of the blower motor. (The rise of the blower motor rating would have to be approximately proportional to the cube of the volume of air flowing through the traction motor.) This means that the losses in the blower and in the blower motor would reach a point at which they would offset the advantages of ventilation and that the efficiency of ventilation near the maximum pos sible volume of air flow would be low. For practical purposes independent forced venti lation is designed to increase the continuous rating of the traction motor to about 65 per cent of the one-hour rating, the simultaneous rise of the one-hour rating being about 5 per cent (see Table 1). When the volume of cooling air blown in or exhausted by the ventilator is V0 (m3/sec) and the effective pressure or vacuum is h (mm of water column), then the rating of the blower motor Pw is calculated from the formula 102 η where η is the resultant efficiency of the blower and the blower motor; the value of η may be assumed as 0.5-0.6. Data obtained under actual working conditions show that the value of h ranges from 16 to 20 mm of water column for traction motors with one-hour ratings up to 50 kW and from 16 to 25 mm of water column for motors with one-hour ratings from 50 to 100 kW. The losses in the traction motor necessary to raise the temperature of 1 m3 of air by #°C are calculated on the assumption that the density of air is 1.293 kg/m 3 , the specific heat of air is 0.237, and the electrical equivalent of a calorie is 4.2W sec. Then N= 1293x0.237x4.2 Û = 1280 ê Wsec. (3.3) Assuming further that the rise of temperature of the cooling air is # = 20°C, we have N=25.6 kWsec. (3.4)
Underground Electric Haulage
48
This means that for every 25.6 kW of losses in the motor the stream of cooling air must be 1 m3/sec, if the rise of temperature is to be 20°C. In traction motors used on the surface the temperature rise is usually 40-50°C, but to keep on the safe side lower rises in temperature of only 20-30°C are accepted for locomotives working under ground. On passing through the blower the temperature of air rises by about 3-5°C. This circumstance makes exhaust ventilation more efficient than forced ventilation: in the former case the temperature of the cooling air reaching the interior of the traction motor is the same as the ambient temperature. The following example shows how to calculate the performance of an independent blower for cooling a traction motor. One-hour rating of the traction motor with no ventilation Continuous rating of the traction motor with no ventilation One-hour efficiency Continuous efficiency
Ph = Pw = ηΗ = η^ =
41.5 kW 19 kW 0.88 0.87
From the data in Table 1 we can assume that independent forced ventilation in creases the one-hour rating of the traction motor by 5 per cent. Then Ph{v) = 41.5x1.05 = 43.6 kW. The continuous rating can then be calculated approximately as Poo(v) = 0.55 PhM = 0.55x43.6 = 25 kW. The efficiency of the traction motor will then be Vm
=0.88,
ifeo(,) = 0.90. The total losses in the output of the traction motor are therefore
AP
^ = (-^~l) p^ = ( w -l)2S = 2 · 7 5
kw
·
The volume of cooling air that must be passed through the traction motor to carry off this amount of heat—see eqn. (3.4)—is calculated as Tz
2 7 5
·
^o = "2^-g- ^ 0 . 1 1 m3/sec,
when it is assumed that the rise of temperature is 20°C. When h = 20 mm of water column and η = 0.5, the output of the ventilator is calculated from formula (3.2): Fw
~
_ 0.11X20 102x0.5 " ü e ü 4 i
kW
·
As is to be seen forced ventilation from an independently driven ventilator substantially increases the output of the traction motor. The input power of the blower motor, calculated as the percentage of the one-hour input of the traction motor, is ,™ 0.043 Λ1 Pw = 100 -TfT" ~ ° · 1
per
cent
Traction Motors
49
The temperatures to which motors and their different parts can be allowed to rise depend on the nature of the winding insulation. These temperatures are defined in electrical engineering standards (see Tables 3, 4, and 5). TABLE 3. PERMISSIBLE TEMPERATURE RISE OF TRACTION MOTORS ACCORDING TO BRITISH (B.E.S.A.) AND AMERICAN (A.I.E.E.) STANDARDS
Method of Type of enclo determination of tempera sure ture
Item
B<2)
Ad)
B<2)
85
105
80
95
65
80
110
130
95
115
90
105
75
90
Ventilated
Thermo meter
90 B.E.S.A. 80 A.I.E.E.
95
65
80
Totally enclosed
Thermo meter
95 B.E.S.A. 90 A.I.E.E.
105
75
90
Ventilated
Thermo meter
95
110
80
95
Thermo Totally enclosed meter
105
120
90
105
Thermo meter Resistance
Totally enclosed
Commutator
Ad)
Continuous rating
120
Ventilated
Cores and mechanical parts in contact with or adjacent to insula tion
One-hour rating
100
Resistance
Armature and field windings
Limiting temperature rise #°C
Thermo meter
(î) "A" denotes "Class A" insulation, which is cotton, silk, paper and similar organic materials when impregnated ; also enamel as applied to conductors. (2) "g" denotes "Class B" insulation, which is inorganic materials, such as mica and asbestos in built-up form combined with binding substances. If Class A material is used in small quantities in conjunction for structural purposes only, the combined material may be considered as Class B, pro vided that the electrical and mechanical properties of the insulated windings are not impaired by the application of the temperature permitted for Class B material. TABLE 4. PERMISSIBLE TEMPERATURE RISE (DEGREES C) OF TRACTION MOTORS ACCORDING TO INTERNATIONAL STANDARDS
Continuous rating
measured measured measured measured with resist with thermo with resist with thermo meter ance method meter ance method
Item
Armature and field windings
One-hour rating
Class A insulation
85
65
100
75
Class B insulation
105
85
120
95
—
85
—
90
Tie rings and com mutators
Notes
ambient tempera ture 25°C
50
Underground Electric Haulage
TABLE 5. PERMISSIBLE TEMPERATURE RISE (DEGREES C) OF TRACTION MOTORS ACCORDING TO POLISH
STANDARD PN-57/E-06001
Continuous rating Measured with resis tance me thod
Item
Ambient Armature winding Field winding
Measured with thermo meter
temperature
Class A insulation*2) Class B insulation Class A insulation*2* Class B insulation
Short-time duty (one-hour rating) Measured Measured with resis tance me with thermo meter thod
25001>
85
65
100
75
120
105
120
105
85
65
100
75
130
100
130
100
90
Commutator
90
w If the temperature of cooling air is higher than 25°C the permitted temperature rise of the particular parts of the motors must be accordingly reduced. <2> The classes of insulation according to Polish Standard PN-55/E-06000 are: Class A insulation is cotton, silk, paper and other organic materials saturated with a binding substance or permanently immersed in a liquid dielectric; Class B insulation is mica, asbestos and similar inorganic materials in built-up form combined with binding substances.
According to the definitions set forth in the Polish Standards PN-55/E-06000 and PN-57/E-06001 the rise of temperature equals the difference between the temperature of the particular part of the motor and the temperature of the cooling air when the work of the motor is continuous or intermittent, or the difference between the temper ature of the particular parts of the motor as measured at the beginning and the end of the period of work when the motor is running on cycles of short duration. Polish Standard PN-57/E-06001 specifies that measurements of the temperature rise made with a thermometer may be accepted only as approximate. So far as the permissible temperatures are concerned the most important parts of the motor are the windings. The rise of temperature (in degrees Centigrade) of the windings can be calculated from the measured resistance according to the formula M
= ^L_J?£ = (234.5+0 c )-(# w -0 c )
where Δ# = the temperature increase to be calculated, êc = the temperature of winding when cold, &m = the temperature of cooling medium, Rc = the resistance of winding when cold, Rh = the resistance of winding when hot. In most cases u
m —
u
c·
Traction Motors
51
3.3. WEIGHTS AND RATINGS OF TRACTION MOTORS
The weight of traction motors used for driving electric mine locomotives for under ground duties ranges from about 300 to about 1000 kg. The weight of the motor depends on its rating and its speed, i.e. on the torque. Electric locomotives of the same weight may be fitted with lighter, lower-rating motors or with heavier, higher-rating motors. Generally, mine locomotives of European make are fitted with motors having lower-ratings per ton of the mine locomotive weight than mine locomotives of American make. The ratio of the rating of motors to the locomotive weight for mine locomotives of European make varies from 4.5 to 6.9 kW per ton at one-hour rated speed of 13 km/hr, while the same ratio for mine locomotives of American make varies from 7.3 kW per ton at one-hour rated speed of 13 km/hr to 14.5 kW per ton at one-hour rated speed of 19 km/hr. The surplus power of the motors allows greater acceleration and more rapid start ing, but also has certain disadvantages as, for example, the slipping of wheels on the rails.